The present invention relates to high accuracy positioning and measuring systems of the type employed to control the location and translation of a tool relative to a workpiece. More particularly, the invention relates to a laser interferometer system that is used for positioning a photohead relative to film on a work table.
U.S Pat. No. 3,884,580 having the same assignee as the present invention discloses an interferometer system that is used for position feedback in a high-accuracy positioning control for a photoplotter or other automatic positioning equipment. Laser interferometers are ideally suited for position control because of their high accuracy. However, due to the expense of the optical components that are employed in laser systems, their use is generally reserved to systems where accuracies of ten thousands of an inch or less are required. A photoplotter employed to produce masks for microcircuits is one example of equipment that requires such accuracy.
In plotters and other devices which operate on film or other workpieces having at least two principal dimensions, the photohead or other tool must be accurately positioned relative to the workpiece along two coordinate axes. In the prior art systems having dual axis positioning devices, two interferometers and two elongated mirrors are required so that the measuring axis of each interferometer is at all times intersecting a reflective surface. The length of the mirrors in each coordinate direction must be at least as long as the maximum span of the work along those axes, and preferably somewhat greater, and with large spans very costly, heavy and delicate mirrors were required.
It is accordingly an object of the present invention to provide a dual axis interferometer measuring system that utilizes only a single elongated mirror for one of the axes, generally the axis having the smaller displacement, and a small reflector for the other axis without substantially diminishing the accuracy otherwise available from interferometer measuring systems.